The present invention relates to a sensor for detecting the concentration of oxygen in exhaust gas emitted from an internal combustion engine of an automobile or other vehicle. The present invention also relates to a process for producing such an oxygen sensor.
Most modern automobiles are designed so that the composition of exhaust gas from engines is kept constant by monitoring the concentration of oxygen in the exhaust gas. The data obtained from such monitoring on oxygen concentration is fed back to a fuel injection system, which in response controls the amount of fuel jetted into the engine so as to maintain a cleaner exhaust gas.
The concentration of oxygen in the exhaust gas is conventionally measured by a device called an oxygen sensor. The prior art oxygen sensor has a solid electrolyte (e.g., zirconia, ZrO.sub.2) body as the base material and includes electrodes, such as those made of platinum, on both sides of the base, with a porous ceramic protective layer such as one made of magnesia-alumina (MgO.Al.sub.2 O.sub.3) spinel being provided on the electrode surface which is in contact with oxygen gas.
Such a prior art oxygen sensor, however, experiences a gradual drop in sensing performance during service. This is believed due to lead poisoning of the electrodes wherein lead in the exhaust gas from the fuel reacts with the sensor electrodes to produce a condensation layer that deprives the electrodes of electrical conductivity.
In order to prevent such lead poisoning of the oxygen sensor, attempts have been made to use thicker electrodes. However, as the thickness of the electrode is increased, the response time, which is one important parameter of the sensor, is increased unless the sensor is subjected to complicated treatments such as aging with an actual exhaust gas.